Field of the Invention
The invention relates to aluminum-copper-lithium alloy rolled products, and more particularly to such products, their manufacturing processes and use, designed notably for aeronautical and aerospace engineering.
Description of Related Art
Rolled products made of aluminum alloy are developed in order to produce fuselage components intended notably for the aeronautical and aerospace industry.
Aluminum-copper-lithium alloys are particularly beneficial for the production of this type of product.
U.S. Pat. No. 5,032,359 describes a vast family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, in particular between 0.3 and 0.5 percent by weight, makes it possible to increase the mechanical strength.
U.S. Pat. No. 5,455,003 describes a process for manufacturing Al—Cu—Li alloys that have improved mechanical strength and fracture toughness at cryogenic temperature, in particular owing to appropriate strain hardening and aging. This patent particularly recommends the composition, expressed as a percentage by weight, Cu=3.0-4.5, Li=0.7-1.1, Ag=0-0.6, Mg=0.3-0.6 and Zn=0-0.75.
U.S. Pat. No. 7,438,772 describes alloys including, expressed as a percentage by weight, Cu: 3-5, Mg: 0.5-2, Li: 0.01-0.9 and discourages the use of higher lithium contents because of a reduction in the balance between fracture toughness and mechanical strength.
U.S. Pat. No. 7,229,509 describes an alloy including (% by weight): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn, 0.4 max Zr or other grain-refining agents such as Cr, Ti, Hf, Sc, and V.
US patent application 2009/142222 A1 describes alloys including (% by weight), 3.4% to 4.2% Cu, 0.9% to 1.4% Li, 0.3% to 0.7% Ag, 0.1% to 0.6% Mg, 0.2% to 0.8% Zn, 0.1% to 0.6% Mn and 0.01% to 0.6% of at least one element for controlling the granular structure. This application also describes a process for manufacturing extruded products. US patent application 2011/0247730 describes alloys including (% by weight), 2.75 to 5.0% Cu, 0.1 to 1.1% Li, 0.3 to 2.0% Ag, 0.2 to 0.8% Mg, 0.50 to 1.5% Zn, and up to 1.0% Mn, with a Cu/Mg ratio between 6.1 and 17, this alloy being insensitive to work hardening.
Patent application CN101967588 describes alloys of composition (% by weight) Cu 2.8-4.0; Li 0.8-1.9; Mn 0.2-0.6; Zn 0.20-0.80, Zr 0.04-0.20, Mg 0.20-0.80, Ag 0.1-0.7, Si<0.10. Fe≤0.10, Ti≤0.12.
The required characteristics for aluminum plates intended for fuselage applications are notably described, for example, in patent EP 1 891 247. It is notably desirable that the plate has a high yield stress (to resist buckling) as well as high fracture toughness in plane strain, notably characterized by a high value of apparent stress intensity factor at break (Kapp) and a long R-curve.
Patent EP 1 966 402 discloses an alloy comprising 2.1 to 2.8% by weight of Cu, 1.1 to 1.7% by weight of Li, 0.1 to 0.8% by weight of Ag, 0.2 to 0.6% by weight of Mg, 0.2 to 0.6% by weight of Mn, a quantity of Fe and Si less than or equal to 0.1% by weight each, and inevitable impurities with a content less than or equal to 0.05% by weight each and 0.15% by weight in total, the alloy being substantially free of zirconium, particularly suitable for obtaining recrystallized sheets.
For certain fuselage applications, it is particularly important that the fracture toughness is high in the T-L direction. Indeed, a large part of the fuselage is designed to withstand the internal pressure of the airplane. As the longitudinal direction of the sheets is generally positioned in the direction of the length of the airplane, they are subjected to stress in the transverse direction by the pressure. The cracks are thus subjected to stress in the T-L direction.
Obtaining high fracture toughness, notably in the T-L direction is particularly delicate on the sheets such as those with thickness between 0.5 and 3.3 mm.
It is known from patent EP 1 891 247 that for plates with thickness between 4 and 12 mm, it may be advantageous that the microstructure be completely unrecrystallized. However, the effect of the granular structure on the properties may be different at different thicknesses. Also, obtaining a substantially unrecrystallized structure for sheets with thickness between 0.5 mm and 3.3 mm is difficult because the energy stored during cold working most often leads to recrystallization during the solution heat treatment. Thus, the sheets with thickness between 0.5 mm and 3.3 mm described in EP 1 891 247 have a 100% recrystallized structure (also see patent FR 2 889 542, Table 6). US patent application 2012/0055590 mentions obtaining an unrecrystallized structure for sheets of 2 mm thick. However, the method proposed in this application to obtain an unrecrystallized structure requires significant cold working, at least 25%, after solution heat treatment and quenching of the sheet. This type of cold working can be delicate to achieve because the sheets reach a high degree of hardness within a few hours after solution heat treatment and quenching. Moreover, significant cold working after solution heat treatment and quenching affects the granular structure, and the products obtained by the method described in US application 2012/0055590 have numerous shearing bands passing through several grains, as shown in FIGS. 11b to 11e, which can notably have negative effects on formability and fracture toughness in certain loading directions or on the location of the corrosion.
Patent EP 1 170 394 also mentions obtaining unrecrystallized structures, but for sheets of thicknesses greater than 3.5 mm.
There exists a need for sheets, of thickness 0.5 to 3.3 mm, made of aluminum-copper-lithium alloy presenting improved properties as compared with those of known products, particularly in terms of fracture toughness in the T-L direction, static mechanical strength and corrosion resistance properties, while having low density. Furthermore, there exists a need for a simple and economical process for obtaining these sheets.